A recent report at the Experimental Biology meetings in 2012 by Reidy et al., is worthwhile because it challenges the monocular view of the importance of single protein sources in sports nutrition. The latter concept has become pervasive despite or because of its simplicity. Perhaps it is time to recap the various factors at play here and take a moment to consider the history behind where we are now. Valuable lessons may have been lost with the passage of time.
Protein in sports performance has its roots in infant nutrition, which is a state of remarkable anabolic needs. By understanding the evolution of infant nutrition one can better appreciate where we are today in sports nutrition. Firstly, it is important to note that all mammalian milks are protein blends, they are not a single protein source and even broad classifications are comprised of numerous protein forms. The first major breakdown is the amounts of casein and whey. The relative amounts of casein and whey vary between species, as well as the total protein content. In human milk it is 40% casein and 60% whey. On the other hand, bovine milk is 20% whey and 80% casein. There are numerous proteins that make up these broad classifications and there are substantial interspecies differences in the form and functionality e.g., human casein does not form a true curd and bovine whey has allergenicity issues largely due to alpha-lactalbumin and beta-lactoglobulin. To explain in simple terms casein is acid insoluble (forms the white part of milk) and whey is soluble and was once called milk plasma (and is yellow in color like plasma). Think “Little Miss Muffet”.
Early infant formulas were largely recreations of cow’s milk, with a comprehensive switch to whey dominant formulas (to mimic human milk) not occurring until the 1990’s. Hydrolyzed proteins were used to reduce allergenicity and not the current sports performance dogma of increasing rate of absorption or anabolism. In extreme cases elemental (amino acid only) formulas were necessary although this leads to atrophy of the gut mucosal lining. An early alternative approach to intolerance was the use of soy proteins, which afforded for many infants a nutritional alternative for those with cow’s milk protein allergies and lactose intolerance (or both).
Thus growth and anabolism has its roots in protein blends and alternatives were built around the need to reduce intolerance and allergy, and not to enhance anabolism through rapid absorption. So why does nature use protein blends? The answer is for much the same reasons that the study in question eludes to – to provide a sustained source of amino acids to promote muscle health and development. However, it is more complex than that. Casein, especially bovine casein forms a curd and this limits gastric emptying. As a result there is an increased opportunity for gastric acid to kill microbial contaminants ingested with the milk. This is actually quite important for virtually all four-legged mammals as the udder is underneath the anus, and if anyone has been to a dairy farm then they will appreciate that lactation and defecation are often simultaneous. This is not an issue with the evolutionary trend to a bipedal existence, and as a result the protein form of milk switched from casein dominant to whey dominant. But it remains a blend.
Soy protein has been embroiled in misperceptions largely because of confusion centered on isoflavones that possess estrogenic activities. Soy isoflavones technically are weak partial agonists, so in the context of no background estrogens they can promote a weak estrogenic response, but all partial agonists can act as antagonists in the presence of full agonists. In part because of this receptor-transduction activity, which is a well-appreciated fundamental of drug discovery, soy is associated with a reduced incidence of estrogen-driven cancers (breast, endometrial etc.). Additionally, soy isoflavones have interesting and desirable actions on transcription factors and can restore balance to redox-sensitive transcriptional events that dictate overall muscle growth e.g., by limiting the catabolic effects of inflammation. Inflammation switches off anabolic – repair genes and limiting inflammation maintains their activity. However, soy isoflavones are not proteins, they are polyphenols – they are distinct from soy proteins but present in soy. Soy proteins and peptides have true potential for cardiovascular health. Lowering cholesterol and improving vascular lipid profiles are neglected benefits of soy proteins in the sports performance mindset.
The final aspect that superficially is the most relevant to the Reidy et al. study is the link between timing of protein digestion and plasma levels of amino acids. Proteins which possess different rates of digestion deliver amino acids to muscle tissue for an extended period of time when compared to amino acids, protein hydrolysates or rapidly digested proteins like whey. The advantage here is that the signals and building blocks for anabolism are sustained. A longer lasting signal may be more desirable than a brief, albeit strong burst of amino acids.
From an evolutionary perspective mammalian milk shares this conceptual advantage. The question that is worth revisiting is “Have we in our desire to create simple messages lost our appreciation of the importance of sustained elevations of amino acids to promote muscle growth and repair?” Does a revisit to evolutionary & historical perspectives help us regain this focus and lead to more desirable outcomes?
- Reidy P.P. et al. Effect of protein blend vs whey protein ingestion on muscle protein synthesis following resistance exercise.